Western diets damage gut microbiota over generations, in ways hard to reverse

Once it's depleted, the garden of microbes that colonize the gut may not regenerate--even with improved diet--in the next generation, says new research. Shown here, lactobacillus, considered to be helpful when it's in plentiful supply in the gut. (ASM Microbe Library)

It may take more than a tub of yogurt to reverse the effects that a high-fat, low-fiber diet have wrought in the bellies of men and women in the industrialized world, says new research.

Indeed, the depletion of gut microbes that comes with diets deficient in fiber extend well beyond the lives of those whose dietary choices made it happen, a new study finds. Over generations of exposure to diets low in fiber, the research shows that a microbiotic population die-off threatens to drive some of the trillions of microorganisms that live in healthy human guts to the brink of extinction.

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FOR THE RECORD

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A reference to trillions of species in the human gut has been corrected in this post. The gut is host to trillions of microorganisms, but to fewer taxa of those organisms.

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And just as in the world of larger plants and animals, when the population of a given gut bacterium falls below a certain level, it's as good as gone, the new research suggests. The reintroduction of more dietary fiber, and the frantic hawking of probiotic powders, may not be enough to bring all the endangered microbiotic taxa back and restore gut health to successive generations.

The new research, published Wednesday in the journal Nature, was conducted on laboratory mice whose guts were deliberately colonized with a wide array of microbes from humans. Researchers from Stanford, Harvard and Princeton universities fed a generation of lab mice a diet very low in microbiota-accessible carbohydrates -- nutrients that are plentiful in the diets of agrarian hunter-gatherers but not in diets that are common in industrialized societies.

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The results were stark enough in the mice, whose guts, collectively, were colonized by an increasingly impoverished population of bacteria, viruses and protozoa. They then fed four successive generations of mice a diet that was low in microbiota-accessible carbohydrates. Even when they put parent generations back on a high-fiber diet, the dearth of microbial diversity in the guts of younger generations became ever starker.

Even when the researchers switched subsequent generations back to a high-fiber diet, the shift failed to restore the microbiotic diversity that had originally flourished in the guts of their ancestor generations.

The findings suggest that, when diseases arise from a depleted gut microbiome, it may take more than a course of probiotics or a daily tub of yogurt to manage those diseases.

The findings demonstrate "a diet-induced ratcheting effect" in which species of microbiota "are not effectively transferred to the next generation," the researchers wrote. Bacteria belonging to the Bacteroidales family were particularly prone to inter-generational die-offs that couldn't be reversed with intentional re-introduction of a diet high in fiber.

There are no "charismatic megafauna"--the equivalent of tigers and elephants--among the trillions of microbes that colonize the gut. (Suffice it to say, there won't likely soon be campaigns to "save the Bacteroidales.") Indeed, the microscopic population of the human gut is so large and diverse, scientists are far from fully characterizing what role individual taxa play in health. But there's clear and growing evidence that species diversity in there is a key factor in digestive, metabolic and even immune health, and when that diversity takes a hit, some aspect of health is sure to suffer.

The authors of the latest study warn that their data hint that "further deterioration of the Western microbiota is possible," as generational changes drive some taxa closer to the brink. The results of doubling down on diets that pose a threat to the gut's microbiotic diversity could be downright apocalyptic, they suggest.

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"Microbiota can change on a timescale that is much faster than the host," wrote the team, led by Erica D. Sonnenburg and Justin L. Sonnenburg of the Stanford University School of Medicine. That fact makes it possible that if dramatic forces--including perhaps a wholesale abandonment of diets rich in fiber--wreak abrupt changes in populations of gut microbiota, the resulting changes "cannot be accommodated by our human biology."